This application claims priority of Japanese application No. 2000-257694 filed Aug. 28, 2000, the complete disclosure of which is hereby incorporated by reference.
The present invention relates to a method of producing through-holes in an aromatic polyimide film having on its surface a laminated or deposited metal film.
Aromatic polyimide films show good high temperature resistance, good chemical properties, high electrical insulating property, and high mechanical strength, and therefore are widely employed in a variety of technical fields. For instance, an aromatic polyimide film is favorably employable in the form of a continuous aromatic polyimide film/metal film composite sheet for manufacturing a flexible printed circuit board (FPC), a carrier tape for tape-automated-bonding (TAB), and a tape of lead-on-chip (LOC) structure.
The aromatic polyimide film/metal film composite sheet can be produced by bonding a polyimide film to a metal film using a conventional adhesive such as an epoxy resin. However, due to low heat-resistance of the conventional adhesive, the produced composite sheet cannot show satisfactory high heat-resistance.
For obviating the above-mentioned problem, a variety of bonding methods have been proposed. For instance, an aromatic polyimide film/metal film composite sheet is manufactured by producing a copper metal film on an aromatic polyimide film by electroplating or vacuum deposition. Otherwise, an aromatic polyamide acid solution (i.e., a solution of a precursor of an aromatic polyimide resin) is coated on a copper film, dried, and heated to a high temperature for producing an aromatic polyimide film on the copper metal film.
An aromatic polyimide film/metal film composite sheet also can be produced using an intermediate thermoplastic polyimide layer which is arranged between the polyimide film and the metal film.
A flexible printed circuit board (FPC) is generally provided with a number of through-holes for electrically connecting a circuit on one side and a circuit on another side. Heretofore, the through-holes have been provided in an aromatic polyimide film by a punching procedure. The punching is appropriate for providing a through-hole having a diameter of 500 xcexcm or more. Due to the recent development of electronic devices, however, it is required that the flexible printed circuit boards or other electronic devices have through-holes having a less diameter such as 300 xcexcm or less.
Japanese Patent Publication H7-109836 describes that a tape carrier composed of a metal conductive layer and a polyimide resin layer is etched on its polyimide resin layer using a laser beam of ultraviolet ray region (400 nm or shorter). In the working examples, the tape carrier is composed of a copper foil of 35 xcexcm thick and a polyimide resin layer of 50 xcexcm thick which is directly produced on the foil.
According to the study of the present inventors, it has now been discovered that commercially available aromatic polyimide film/metal film composite sheets are not appropriate for producing through-holes therein utilizing the laser etching procedure. For instance, a representative commercially available aromatic polyimide film/metal film composite is manufactured by coating a solution of a polyimide precursor (i.e., polyamide acid) on a metal film and subsequently heating the coated metal film to convert the polyimide precursor into a polyimide. In this procedure, the heating is performed at not so high temperatures, so as to not deteriorate the metal film by the heating. Accordingly, an employable polyimide precursor is limited to that capable of being turned into polyimide at a relatively low temperature. According to the study of the inventors, the polyimide manufactured from the polyimide precursor capable of being turned into polyimide at a relatively low temperature cannot give through-holes of good conditions concerning the smoothness of the inner surface of the through-hole and the shape of the through-hole.
For instance, the through-hole produced in the polyimide film which is prepared from a polyimide precursor capable of being turned into polyimide at a relatively low temperature is liable to have a cracked inner surface having a section illustrated in FIG. 1-(1) in which each of 11 and 13 stands for a metal film and 12 stands for a polyimide film. The through-hole 14 having cracked inner surface is turned into a through-hole having varying diameters after it is subjected to desmear processing, as illustrated in FIG. 1-(2).
It is an object of the invention to provide an improvement of a method of producing a through-hole in an aromatic polyimide film having on its surface a laminated or deposited metal film by applying a laser beam onto the polyimide film.
The present invention resides in a method of producing a through-hole in an aromatic polyimide film having on a surface thereof a laminated or deposited metal film by applying laser beam onto the polyimide film, which is characterized that the polyimide film comprises a high Tg aromatic polyimide layer which has a glass transition temperature of 300xc2x0 C. or higher or has no noticeable glass transition temperature and is produced from a biphenyl-tetracarboxylic acid compound and an aromatic diamine compound and a low Tg aromatic polyimide layer which has a glass transition temperature of lower than 375xc2x0 C. but higher than 200xc2x0 C., provided that the glass transition temperature of the low Tg polyimide layer is lower than the glass transition temperature of the high Tg polyimide layer and the metal film is laminated or deposited on the surface of the low Tg aromatic polyimide layer on the side not facing the high Tg aromatic polyimide layer.
The invention further resides in a method of producing a through-hole in an aromatic polyimide film having on a surface thereof a laminated or deposited metal film by applying a laser beam onto the polyimide film, which is characterized in that the polyimide film comprises a high Tg aromatic polyimide layer which has a glass transition temperature of 300xc2x0 C. or higher or has no noticeable glass transition temperature and is produced from a biphenyltetracarboxylic acid compound and an aromatic diamine compound and two low Tg aromatic polyimide layers each of which has a glass transition temperature of lower than 375xc2x0 C. but higher than 200xc2x0 C., provided that the glass transition temperature of the low Tg polyimide layer is lower than the glass transition temperature of the high Tg polyimide layer and the high Tg polyimide layer is placed between the low Tg polyimide layers.